Multicarrier communications systems, such as community antenna television systems (CATV), use coaxial cable to distribute standard TV signals to homes or establishments subscribing to the service. The TV signals may originate at a distant broadcast station or studio, or may be locally generated within the facilities of the CATV system. A specially designed antenna system (community antenna), which provides freedom from noise, interference, and multipath distortion not obtainable directly at the subscribers' homes, may be used to pick up the TV signals generated at remote locations.
A typical CATV system comprises four main elements: (1) a head end, in which the signals are received and processed; (2) a trunk system, the main artery carrying the composite signal; (3) a distribution system, which is bridged from the trunk systems and carries signals to subscriber areas; and (4) subscriber drops, fed from taps on the distribution system to feed into the subscribers TV receiver. Each TV signal or channel modulates a dedicated carrier frequency which carries the TV signal over the cable system (e.g. linear lightwave transportation and coaxial cable transportation) to the subscribers. The modulated carriers, however, are combined at the headend into a composite multicarrier signal that is distributed to the subscribers of the system (hence the name multicarrier system).
In a typical CATV multicarrier system, it is not uncommon for a composite signal to experience losses on the order of 20 dB across a kilometer of coaxial cable. As a result, in modern systems the composite signal is distributed on linear lightwave trunks to a fiber node from which the coaxial portion of the distribution network originates. In the coaxial portion of the network, the composite signal is typically amplified by up to 4 coaxial amplifiers before it reaches the customer's home. The linear lightwave subsystem together with the coaxial amplifiers introduce nonlinearities into the system. The resulting distortion causes substantial losses in dynamic range of the composite signal. That is, the presence of the linear lightwave transmitter and receiver, and the coaxial amplifiers, causes the ratio of the signal to the noise and distortion of the transmission to degrade with distance.
Heretofore, there have been several proposed solutions to the problems associated with the nonlinear distortions and limited dynamic range of such multicarrier, multichannel systems. One solution was disclosed in U.S. Pat. No. 3,898,566, issued to Switzer et al. on Aug. 5, 1975 (hereinafter Switzer). Switzer asserted that the nonlinear distortions of the system caused intermodulation distortions within the band of carrier frequencies of the composite signal. From this, Switzer further asserted that the intermodulation distortions caused the composite signal to substantially degrade over relatively short distances, and that as the number of channels on the system increase, the effects of intermodulation distortions also increased. That is, Switzer asserted that system nonlinearities enabled intermodulation distortion to limit the dynamic range of the composite signal and the number of channels that could be provided on the system. Thus, Switzer concluded that in order to provide more channels in one composite signal on the system, without substantially degrading system performance, the intermodulation distortion would have to be significantly reduced.
To reduce the intermodulation distortion, Switzer proposed making the multicarrier system harmonically related and coherent. That is, a system in which all carriers have frequencies that are integral multiples of a common fundamental frequency (harmonically related), and all carrier frequencies have phases that are adjusted or tuned with respect to each other (coherent). Switzer asserted that a composite signal composed of such harmonically related and coherent carrier frequencies reduces the peak to peak amplitude of the composite signal, and thus reduces the non-linear intermodulation distortions that degrade the composite signal. Basically, Switzer found a set of phases for a given set of carrier frequencies by cutting the lengths of the cables for each carrier so that, when combined into the composite signal, the carriers have a set of phases or a phase relationship with one another. Switzer asserted that the set of phases would reduce the peak-to-peak voltage of the composite signal, and thus reduce the intermodulation distortion that degrades the composite signal.
The phase adjustment method described in Switzer, however, is essentially ad hoc and relies on only one variable, the peak voltage of the composite signal, to determine the phase for each carrier in the system. This presents a problem of dimensionality for many large systems. That is, it is difficult to optimize a phase configuration in a system having many degrees of freedom (i.e. N channels) with only one parameter (peak voltage) to guide the process. Thus, the Switzer method presents a problem of practicality.
In addition, the Switzer method fails to consider the various effects on interchannel timing drifts and stability. This includes thermal effects, component aging and power variations between carriers. That is, even if Switzer sets the carriers to an initial set of optimum phases, the phases of each individual carrier would have to be periodically recalibrated to insure that the initial set of phases is maintained over time (i.e. the zero crossing of all the carriers must stay aligned by periodically adjusting for any phase offsets of the carriers). Switzer does not disclose nor claim any such realignment or readjustment to keep the phases aligned over time.
Moreover, Switzer failed to consider the effects of the intermodulation distortion on frequencies outside the band of frequencies of the multicarrier system. That is, in obtaining the set of phases for a given set of carrier frequencies, the Switzer method is directed to minimizing the intermodulation distortions on frequencies in the band of carrier frequencies. The effects of intermodulation distortion of the composite signal on out-of-band signals or frequencies (which may be of digital format) is not considered or addressed. Thus, the Switzer method does not insure a set of phases for a given set of TV carrier frequencies that would not substantially interfere with the other out-of-band signals being carried on the same transmission medium.
Another method of minimizing the effects of intermodulation distortion in multicarrier communications systems was disclosed in German Patent No. 2,930,659 A1, issued to Wolfgang Krick on Jul. 26, 1979, entitled "Process and Circuit Arrangement For Setting Phase Angles Between Coherent Carriers" (hereinafter Krick). Krick discloses a method for setting the phase angles of a given set of carrier frequencies comprising a composite signal according to a preset pattern.
Basically, Krick provides a method of setting the phases of the carriers so that the carriers have a predetermined phase relationship with respect to each other. That is, setting the phases for a specific set of carriers so that the carrier phases differ from each other according to some preset pattern. The method includes choosing a reference pair of adjacent carriers, and mixing the carriers to obtain an intermediate frequency sinusoid. The phase of this reference intermediate sinusoid is used as a reference phase for setting the individual phases of the carriers.
To set the individual phases of the carriers, a third carrier, being the next adjacent carrier to the reference pair on the frequency spectrum, is mixed with the closest carrier of the reference pair to obtain a second intermediate sinusoid having a second phase. The phase of the third carrier is then continuously shifted until the phase angle between the reference phase and the second phase corresponds to the preset pattern of values 0.degree. and 180.degree. only. This is done for each carrier of the system so that all the carrier phases are adjusted to the preset phase pattern.
As a result, Krick provides a method that requires the continuous monitoring of the intermediate sinusoid phases, and the incremental tuning of the carrier phases to maintain a preset pattern of carrier phases over time. Moreover, as with Switzer, Krick fails to even consider the effects of the predetermined phase pattern on out-of-band intermodulation distortion.
More recently, a method of minimizing intermodulation noise in multicarrier systems was disclosed in an international application published under the Patent Cooperation Treaty (PCT) on Jan. 9, 1992. The patent, entitled "Optimal Signal Synthesis For Distortion Canceling Multicarrier Systems", was issued to Ron D. Katznelson (hereinafter Katznelson). Katznelson disclosed a method and apparatus for minimizing the peak amplitude of the composite signal through a closed-loop feedback scheme.
Generally, Katznelson proposed adding a number of out-of-band auxiliary carriers, that extended up to five times the highest in-band frequency, to the composite signal. It was asserted that by adding these auxiliary carriers, with appropriate amplitudes and phases, the in-band intermodulation distortions products could be substantially minimized.
Although this method could be applied to broad band fiber optic transmission systems, it would not be applicable to present day coaxial cable systems due to bandwidth limitations. Moreover, the method of adding out-of band auxiliary carriers is not useful for systems having mixed analog/digital signal applications, where the out-of-band distortions due to analog signals should be controlled and minimized.
Katznelson also discloses a method of optimizing the phases of the carrier frequencies without the addition of auxiliary carriers. Generally, the object of the method is to reduce the in-band distortions by minimizing the peak amplitude of the composite signal. As in Switzer and Krick, however, the out-of-band distortions were never addressed or quantified.